1. Field
The present application relates to a solid-state image sensor.
2. Description of the Related Art
In general, in an imaging apparatus such as a digital camera, a solid-state image sensor of CCD type or CMOS type is used. For example, a solid-state image sensor of CMOS type has a plurality of pixels arranged in the form of a two-dimensional matrix. For example, a pixel has a photoelectric conversion part such as a photodiode, which generates a signal charge according to the amount of incident light, a transfer transistor, an amplifying transistor, a reset transistor, and a selection transistor. The drain of the transfer transistor is connected to the gate of the amplifying transistor and functions as a floating diffusion region that accumulates signal charges generated in the photodiode.
A pixel signal is output to a vertical signal line provided in each column for each row during the read period. For example, during the read period, the selection transistor in the row to be read turns on. Then, in the pixel in the row where the selection transistor is in the on state, first, the reset transistor turns on for a fixed period of time and charges in the floating diffusion region are reset. Next, the transfer transistor turns on and the signal charges generated in the photodiode are transferred to the floating diffusion region. A signal voltage based on the charges transferred to the floating diffusion region is output to the vertical signal line by the amplifying transistor that operates as a source follower circuit.
In recent years, a solid-state image sensor in which an amplifying transistor, a reset transistor, and a selection transistor are shared by two pixels arranged in the direction of the column is proposed (for example, see Japanese Unexamined Patent Application Publication No. 2007-165864). In this kind of solid-state image sensor, for example, the drains (a part of the floating diffusion region) of the transfer transistors of two pixels that share the amplifying transistor etc. are connected to each other by wiring such as a metal and shared by the two pixels. In general, the wiring that connects the drains of the transfer transistors of two pixels arranged in the direction of the column is arranged parallel to the vertical signal line.
During the read period, when the selection transistor turns on, a current flows to the vertical signal line from a power supply via the amplifying transistor and the selection transistor. Because of this, the voltage of the vertical signal line rises. Then, by the coupling of the vertical signal line and the floating diffusion region, the voltage of the floating diffusion region rises. The voltage of the floating diffusion region also rises by the coupling of the gate of the selection transistor and the floating diffusion region.
In particular, in the solid-state image sensor in which the wiring in the floating diffusion region is arranged parallel to the vertical signal line (for example, a solid-state image sensor in which two pixels share an amplifying transistor etc.), the influence of the coupling of the vertical signal line and the floating diffusion region becomes great and the rise in the voltage of the floating diffusion region becomes large.
When the voltage of the floating diffusion region rises, there is a possibility that the voltage between gate and source of the reset transistor does not become a threshold voltage or more when the gate of the reset transistor is set to a high level. In this case, the reset transistor does not operate normally, and thus the charges in the floating diffusion region are not reset and the operation range of the amplifying transistor deviates from the normal operation range. For example, when the reset transistors of a plurality of pixels do not operate normally, the voltages of the floating diffusion region vary in the plurality of pixels, and as a result of that, the output characteristic of the amplifying transistor vary in the plurality of pixels. In this case, dark shading or fixed pattern noise occurs in an image.